This invention relates, in general, to semiconductor devices, and more particularly, but not limited to, a semiconductor chip electrically interconnected to a substrate.
Flip-chip technology is well known in the art. A semiconductor chip having solder bumps formed on the active or front side of the semiconductor chip is inverted and bonded or attached to a substrate through the solder bumps by reflowing the solder. A solder joint is thus formed between the semiconductor chip and the substrate and a narrow gap formed between the semiconductor chip and the substrate.
One major obstacle to flip-chip technology is the unacceptable reliability of the solder joints due to the mismatch of the coefficient of thermal expansion between the semiconductor chip and the substrate. The substrate is typically comprised of a ceramic material or a polymer composite. In addition, because the solder joints are small, they are subject to failures.
In the past, the solder joint integrity of flip-chip interconnects to a substrate has been enhanced by underfilling the volume between the semiconductor chip and the substrate with an underfill material comprised of a suitable polymer. The underfill material is typically dispensed around two adjacent sides of the semiconductor chip and the underfill material flows by capillary action to fill the gap between the semiconductor chip and the substrate.
As the spacing between each solder bump is reduced, the height of the solder bump is similarly reduced, especially when using semiconductor chips which have solder bumps formed at the periphery of the semiconductor chip. The height of the solder bump creates a very narrow gap between the semiconductor chip and the substrate, approaching less than 50 microns, which can not be adequately underfilled by using existing underfill techniques.
One way to solve the problem is to use an underfill material which readily flows in the narrow gap between the semiconductor chip and the substrate. This underfill material contains less filler material than other underfill materials which do not flow as readily in narrow gaps of 50 microns or less. A problem with this type of material is that it also has an extremely high mismatch of the coefficient of thermal expansion between it and the semiconductor chip, the solder bumps, and the substrate because of the reduced amount of filler material contained therein. It would be desirable to provide an underfill material which has thermal properties which more closely match that of the surrounding materials.
It is also important that the underfill material adhere well to the semiconductor chip, the solder bumps, and the substrate (all of the interfaces) to improve the solder joint integrity. Before bonding the semiconductor chip to the substrate, flux, a chemical such as a rosin, is usually applied to the semiconductor chip and the substrate to free them from oxides and promote the bonding thereof. After bonding, the flux must be removed because any remaining residue from the flux may affect the adhesion properties of the underfill material and may also pose a corrosion problem to the semiconductor chip.
In the past, flux has been removed by an organic solvent or aqueous solution. The flux used in bonding the flip chip to the substrate is hard to remove because of the narrow gap left between the semiconductor chip and the substrate. Thus, remaining residue from the flux has inhibited the bonding of the underfill material to the semiconductor chip and the substrate, causing subsequent delamination of the underfill material. This delamination shortens the fatigue life of the solder joints. Thus, it would be advantageous to provide a method of improving the adhesion between the encapsulation material and all of the interfaces.
Advances in the composition of flux have reduced the amount of residue that is left remaining after use. However, this type of flux may still require cleaning of residue to promote better adhesion at all the interfaces.
In addition, a fluxless removal process of oxides has been used. This fluxless process involves the use of a reducing atmosphere and, if desired, a corrosive gas, to remove oxides. This process leaves no residue, however, it would still be desirable to clean contaminants present on the surface of the semiconductor chip prior to encapsulation.